1. Field of the Invention
The present invention relates to inorganic pigments and to a method for making them. More particularly, the present invention concerns such pigments which are stable at high temperatures and suitable for use in the processing of plastics.
2. Prior Art
As is known to those skilled in the art to which the present invention pertains, most inorganic pigments are not suitable for plastics processing because of their thermal instability at temperatures above 175.degree. C. Thus, the known yellow pigments such as iron oxide, lead chromate and zinc chromate are not well chosen for the yellow coloring of plastics such as polyethylene, polypropylene, polyvinyl chloride, polycarbonates, polyamide and the like. The same is true of other inorganic pigments for obtaining other colors. Furthermore, organic pigments show color degradation at temperatures of about 175.degree. C., thereby precluding their use in the processing of plastics. Thus, the need for pigments stable at high temperatures and capable of giving various desired colors is readily apparent.
As will subsequently be detailed, the present invention provides such stable pigments, ones capable of giving hues which range in color from pale yellow to brown to red.